While preservative treatment significantly prolongs the service life of wood for utility poles, piling, railroad ties and other wood products, treated woods are increasingly becoming the subject of concern at the end of their life cycles. Such treated wood must be eventually removed from service due to fungal decay, insect attack, car damage, or changes in utility service needs. For many years, old treated wood was given away to local property owners for recycling into fence posts and other similar applications, but many utilities have begun to reexamine this practice due to environmental concerns.
The Environmental Protection Agency (EPA) currently regulates disposal of treated wood materials using a Toxicity Characteristic Leaching Profile (TCLP) whereby the treated wood is subjected to evaluation by a leaching test procedure. These treated wood materials are tested because they typically contain biocides, such as pentachlorophenol (PCP), that are regulated under the Resource Conservation and Recovery Act (RCRA) and are perceived as posing a health risk when placed in a land fill. In the leaching test procedure, a portion of the wood is ground and subjected to a chemical extraction. The extract is analyzed for residual chemical content. Detection of certain chemicals above prescribed limits classifies a tested material as a hazardous waste and requires that the material be placed in a hazardous waste landfill site.
The current EPA regulations specify a TCLP limit of 100 ppm for pentachlorophenol. However, proposed changes to the TCLP limit, in order to comply with new drinking water standards, would lower the acceptable level to 0.1 ppm. Most pentachlorophenol treated wood have a TCLP in the range of 7 to 10 ppm, thus these treated wood materials would fail to meet the new TCLP requirements and therefore would have to be classified and treated as hazardous wastes.
For example, there are over 187 million utility poles in the United States and approximately 40% of these poles are treated with pentachlorophenol. A typical utility company replaces approximately 1 to 2% of its wood pole system each year, creating the potential for disposal of nearly 1.5 million pentachlorophenol treated poles every year. In addition, pentachlorophenol has been used to treat wood used for decking, timber, glu-laminated timbers and a variety of other products. Without effective methods for safe disposal for these treated wood products, these treated wood products will need to be shipped to a hazardous waste facility. The affect would be to increase the cost of disposal for treated wood products dramatically and, consequently, increase their cost to users and purchasers. Moreover, increasing the volume of materials going to hazardous waste facilities will accelerate the rate at which such sites reach their maximum capacity levels. As a result, there is a need for effective methods for disposing of wood treated with this chemical.
A number of different strategies have been identified for disposing of treated woods, including incineration, chemical modification or bioremediation. Each of these methods has its drawbacks. The use of incineration for disposing of treated wood materials is highly effective for eliminating pentachlorophenol. It has the added benefit of cogeneration of electrical power and steam. However, the regulatory hurdles in obtaining a license to burn pentachlorophenol treated wood are considerable, and presently there are few sites where incineration is possible.
Chemical modification or treatment employs either a detoxifying or extraction system to remove a target chemical from a waste material, thereby reducing the volume of waste which must be disposed of in a secure facility. Extraction and detoxification systems have been developed for remediating contaminated soils, but there are currently no systems available for handling treated wood.
Bioremediation has been touted as the preferred method for treating contaminants, but there are very few examples where bioremediation has been successfully employed. For the bioremediation of wood, the process involves subdividing or chipping the wood and then placing it into an environment conducive to degradation by fungi or bacteria. The process would ultimately result in decomposition of the pentachlorophenol over a period of several days to weeks, or possibly months. The process employs various types of soil farming and this can be extremely labor intensive. Studies on solid-phase and slurry-phase bioremediation of materials contaminated with pentachlorophenol have revealed that these bio-processes are slow and inefficient, achieving a maximum degradation of about 50%. It was also found that biodegradation was limited to lower molecular weight constituents rather than the more hazardous, higher molecular weight compounds. Consequently, the technology remains largely experimental.
Supercritical fluids (SCFs) have been used in a number of industrial and pollution control processes. SCFs have properties that fall between those of normal gases and liquids. Most pure substances will typically exist in a solid, liquid or gas phase. These phases can exist singly or in equilibrium with other phases. The point when the distinction between gas and liquid disappears is known as the gas-liquid critical point. The temperature and pressure at the critical point are known as the critical temperature (T.sub.c) and the critical pressure (P.sub.c). No gas can be liquified above its T.sub.c regardless of how great a pressure is applied. When a substance is heated above its T.sub.c and compressed beyond its P.sub.c, the substance is said to be in the supercritical fluid state.
In the supercritical state, substances can be used to penetrate solid matrixes in a manner similar to a gas, due to part gas-like and part liquid-like properties. Densities of such fluids approach those of liquids. However, the density of a SCF may be continuously changed without causing a phase separation by changing pressure and/or temperature. SCF viscosities and diffusivities are also intermediate to those of the liquid and gas phases. Density dependent properties, such as solvent power, will also undergo corresponding changes and this can be advantageously used. The properties of SCFs enable them to have the solvent power of liquids but with better mass transfer properties than liquids. Supercritical extraction processes can be designed to yield a wide range of products. The extraction may be used to remove undesirable contaminants from a desired solid matrix product or to separate and isolate a valuable product from a solid matrix waste.
Supercritical fluid technology has been applied in the extraction of hazardous substances from soil and plants, including the removal of bound (nonextractable) pesticides. Recent studies on removal of DDT and polychlorinated biphenyls (PCB) and dioxins have shown that SCFs can remove pesticides from solid matrixes. Carbon dioxide is especially attractive for this use because it is safe, nontoxic, inexpensive and readily available. In the extraction of pentachlorophenol from soil, supercritical carbon dioxide has been found to recover 240% more pentachlorophenol than other solvent extraction methods, e.g., Soxhlet extractor.
SCFs are particularly useful in reducing the volume of toxins to be handled and disposed. The remaining smaller amounts of toxic materials may then be destroyed at a much lower cost through further treatment; for example, by combustion, wet oxidation, SCF oxidation or biodegradation.
Accordingly, it is an object of the present invention to provide a process for remediating pentachlorophenol treated wood using supercritical fluids.
Another object of the present invention is to provide a process for remediating pentachlorophenol treated wood that allows recovery and recycling of the extracted chemicals.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention.